The first tunable lasers were systems using organic dye molecules in solutions. [F. P. Schaefer, W. Schmidt and J. Volze, Organic dye solution laser, App Phys Lett 9 (1966), pp. 306-309.] Although these lasers are still operated today, this technology is often not suited to build up small-sized integrated optical systems.
Presently, only a few highly efficient Cr3+-doped tunable laser crystals have been developed, such as Cr3+:LiS(C)AF (Cr3+:LiSrAlF6 and Cr3+:LiCaAlF6), emerald Cr3+:Be3Al2(SiO3)6 and alexandrite Cr3+:BeAl2O4. These lasers are limited and generally tunable in the 600-1000 nm wavelength range.
One of the first tunable solid-state gain media is the alexandrite host (BeAl2O4) doped by Cr3+ (U.S. Pat. No. 3,997,853 to R. C. Morris et. al.) which has an emission range of 701-818 nm. [J. C. Walling, O. G. Peterson, H. P. Jensen, R. C. Morris and E. W. O'Dell, Tunable CW alexandrite laser, IEEE J Quantum Electron 16 (1980), pp. 120-121.] Today's most frequently used tunable solid-state laser is Ti-sapphire (α-Al2O3) offering emission between 670 and 1100 nm, with maximum intensity at about 800 nm. For Ti3+, wide emission bands are typically due to a vibronical broadening of the 2T2-2E absorption. [P. F. Moulton, Spectroscopic and laser characteristics of Ti:Al2O3, J Opt Soc Am B 3 (1986), pp. 125-133.] Although a Ti-sapphire system shows many advantages, pumping with, for example, flash lamps is not efficient because of the relatively short lifetime of 3 μs is for the upper laser level. Therefore, pumping is commonly performed by a frequency doubled Nd:YAG (Nd3+:Y3Al5O12) laser. Such systems can become rather large and may not be particularly suited for integration.
A laser media family which has a similar tuning range as Ti-sapphire is Cr3+-doped ternary colquiriites, e.g. LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF). [S. A. Payne, L. L. Chase and G. D. Wilke, Optical spectroscopy of the new laser materials LiSrAlF6:Cr3+ and LiCaAlF6:Cr3+, J Luminesc 44 (1989), pp. 167-176; S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway and H. W. Newkirk, Laser performance of LiSrAlF6:Cr3+, J Appl Phys 66 (1989), pp. 1051-1056.] In the case of LiCAF, the peak lasing wavelength is at 780 nm with a tuning range from 720 to 840 nm. [S. A. Payne, L. L. Chase, H. W. Newkirk, L. K. Smith and W. F. Krupke, LiCaAlF6:Cr3+ a promising new solid-state laser material, IEEE J Quantum Electron 24 (1988), pp. 2243-2252.] LiSAF has an even wider tuning range, covering 780-1010 nm. Here, the peak lasing wavelength is at 825 nm. [S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway and H. W. Newkirk, Laser performance of LiSrAlF6:Cr3+, J Appl Phys 66 (1989), pp. 1051-1056.] A broad absorption band around 670 nm allows pumping by laser diodes.
Emerald, Cr3+:Be3Al2(SiO3)6, a trivalent chromium activated beryl, is one of the first room-temperature, frequency tunable laser crystals and has stimulated vibronic solid state laser research. (U.S. Pat. No. 4,464,761, Aug. 7, 1981 to R. R. Alfano et. al.) Emerald, like alexandrite operates in a four level, phonon terminated mode and exhibits gain over a 695-835 nm wavelength range. Its broad fluorescence bandwidth, together with a high gain cross section and 65 μs room temperature fluorescence lifetime, make emerald an excellent laser material for high power, Q-switched, or mode-locked operation. Highly efficient quasi-cw (continuous-wave) laser operation has been achieved in emerald over the 720-842 nm tuning range. [S. T. Lai J. Opt. Soc. Am. B 4 (1987), p. 1286.]
The development of tunable solid state lasers based on the Cr4+-ion began in 1988 with forsterite, Cr:Mg2SiO4. [V. Petricevic, S. K. Gayen and R. R. Alfano. Appl. Phys. Lett. 53 (1988), p. 2590.] It was rapidly extended to other crystalline media, such as Cr4+-doped Y3Al5O12 [N. B. Angert, N. I. Borodin, V. M. Garmash, V. A. Zhiynyuk, A. G. Okhrimchuck, O. G. Siyuchenko and A. V. Shestakov Soviet Journal of Quantum Electronics 18 (1988), p. 73], Y2SiO5 [Deka, B. H. T. Chai, Y. Shimony, X. X. Zhang, E. Munin and M. Bass Applied Physics Letters 61 (1992), p. 2141], Y3ScxAl5-xO12 [S. Kuck, K. Petermann, U. Pohlmann, U. Schonhoff and G. Huber Applied Physics B 58 (1994), p. 153], and Ca2GeO4 [V. Petricevic, A. B. Bykov, J. M. Evans and R. R. Alfano Optics Letters 21 (1996), p. 1750]. The Cr4+-ions in tetrahedral coordination proved to be useful for the realization of room temperature tunable solid-state lasers in the spectral range between 1.1-2 μm. The Cr4+-doped lasers exhibit many favorable spectroscopic characteristics that have enabled their use as versatile solid-state laser sources in the 1.1-1.6 μm wavelength range. These include the presence of broad absorption bands that overlap with the output wavelengths of the already existing commercial pump lasers such as Nd:YAG and Ti:sapphire; a 4-level energy structure that allows continuous-wave (cw) as well as pulsed laser operation with low threshold pump powers; and broad emission bands in the near infrared. The broadest tuning ranges obtained to date with some of the Cr4+ solid-state lasers are shown in Table 1. Use of various hosts enables the generation of tunable radiation to cover the whole wavelength range between 1.13 and 1.63 μm.
TABLE 1Broadest tuning ranges obtained with some Cr4+ lasersCr4+ laserTuning rangeCr4+: forsterite1130-1367Cr4+: YAG1309-1596Cr4+: Y3Sc0.5Al4.5O121394-1628Cr4+: Y3ScAl4O121464-1604Cr4+: Ca2GeO41340-1490